Method of cleaning radioactive articles



3,@07,8l4 Patented Nov. 7, 1961 3,007,814 METHOD OF CLEANING RADIOACTIVE ARTICLES Thomas J. Bulat, Davenport, Iowa, assignor to The Bendix Corporation, a corporation of Delaware No Drawing. Filed Oct. 4, 1956, Ser. No. 613,819

' 13 Claims. ((31. 134-1) This invention relates to methods of removing radioactive material adhering to articles, an operation usually termed decontamination, and is especially directed to a process utilizing ultrasonic waves for that purpose.

The removal of such radioactive material presents serious problems. Substantially complete removal is necessary, since even a small amount of radioactive residue may have harmful cumulative effects. Mechanical removal by scrubbing or the like, a method widely used, requires relatively long and vigorous treatment, and even then is ineffective where the article to be decontaminated has recesses or interstices that cannot be effectively reached in this manner. Such articles must be disposed of in such a way that radiation from them will not be harmful, a wasteful and troublesome operation.

An object of the invention is to provide a novel decontamination method that will remove substantially all adherent radioactive material, substantial removal being intended to mean that, if there should be any residue of such material, its effects will be negligible and inconsequential. A further objective is to provide such a method that will not involve scrubbing or other abrasive operations, and which will be basically mechanical and not manual.

These objects are accomplished in general by the use of ultrasonic cleaning in liquid, since this method has been found to be uniquely effective in separating substantially all of the radioactive material from even the most inaccessible areas of an article, as long as the liquid can penetrate to such areas, and placing such material in suspension in the liquid, so that the decontaminated article can be Withdrawn from the liquid substantially free from radioactivity. This decontamination method can be carried out in a small fraction of the time now necessary for scrubbing methods, removes radioactive material more thoroughly, and can clean articles that now must be discarded and buried.

However, it has been found in practice that when this decontamination method is used radioactive particles are adsorbed from the liquid by the metal container forming part of the ultrasonic cleaning apparatus, requiring decontamination of the container, as by scrubbing. Moreover, special precautions must be taken in disposing of the cleaning liquid carrying radioactive particles. For instance, if such liquid is poured into a pit in the ground which is thereafter filled, a common expedient for disposing of radioactive material, there may be danger that the contaminated liquid will seep down to the water table and contaminate wells or streams.

-An object of the invention is to provide a method of preventing contamination of the container during ultrasonic decontamination of articles in liquid. A related purpose is to accomplish this object by a simple, inexpensive and readily operated procedure.

A further purpose is to provide a novel and improved method of facilitating the disposal of liquid canrying radioactive material.

Another purpose is to prevent the contamination of the indicated container and also facilitate disposal of contaminated liquid by a single method.

These objects and purposes are in general accomplished by providing an inner container for liquid in which the contaminated articles are placed, and which is inserted in the metal container or bowl of a standard ultrasonic cleaner which ordinarily contains the liquid in which articles are ultrasonically cleaned. A suitable example of such apparatus is the ultrasonic cleaner manufactured by Bendix Aviation Corporation, Davenport, Iowa and described in the September 1954 issue of Production Equipment, published by Wilson-Carr, Inc., Chicago, Ill.

The material of which the inner container is made, and the thickness of such material, must be selected so that the ultrasonic waves will pass through the inner container into the liquid therein with suflicient energy to produce adequate cavitation therein. Materials having relatively low impedance to ultrasonic waves and low reflectance at an interface with liquids are required, among them being glass, metals and synthetic resins, comprising metal foils, especially aluminum and copper, and synthetic resin sheet material, particularly polyethylene and polyvinyl resins.

The thickness of the container material must also be selected between a maximum at which the transmittal of ultrasonic waves at normal ultrasonic cleaner energies is impeded to the point where cavitation within the inner container is insuflicient for adequate cleaning, and a minimum at which the material is ruptured by the waves and cavitation. These limits necessarily vary, not only with different types of material but with diflerent kinds of each .maten'al; but for any given species of material for use in a particular ultrasonic cleaner a suitable thickness can be selected.

An important feature of the invention is the use of flexible material for the inner container, since the latter can be readily closed and sealed after the cleaned articles are removed, greatly facilitating safe disposal of the contaminated liquid.

Flexible sheets of synthetic resins and of metal foils have been found to be especially suitable for the inner container, and standard commen'cal polyethylene has been successfully used. The minimum thickness of such synthetic resins is determined by the ability to support the liquid without breaking, this limit being above that at which the resin would be affected by cavitation, since flexible material of this type is not readily ruptured by cavitation. However, it has been found that with thicknesses over one-sixteenth of an inch the loss of ultrasonic energy is appreciable, and the thickness of the inner container material should be below that figure.

Inner containers of this type can be supported in the container or bowl of the cleaner in various ways, one simple arrangement being to use an inner container of about the same top circumference as the outer container top, and bend the margin of the inner container outwardly over the top of the outer container. This is especially effective when the inner container material is somewhat elastic.

When metal foil is used, its thickness likewise should not be greater than one-sixteenth of an inch in order to avoid objectionable loss of energy. With foil there is more danger of rupture by cavitation; and when standard commercial aluminum foil is used, a thickness greater than five thousandths of an inch should be used. Other metal foils that may be employed include lead, copper and tin foils. The tin or copper foil should not be less than one-thousandth of an inch thick, and neither should be more than one-sixteenth of an inch thick. Ordinary commercial grades of these foils may be used. However, the foil thickness will of course be adequate to avoid rupture by the inserted articles or by handling during disposal.

It is advantageous when using flexible inner containers v to have them conform generally in size and shape to the tainer will be greatly reduced if the space between the containers is filled with liquid. V

The liquid detergent will be selected in accordance with the character of the contaminants following established practicein ultrasonic cleaning in liquids. It maybe water, an-aqueous solution ofdetergent substancesgor a liquid of a different character, which may be basically.

a petroleum derivative.

In carrying out the process the inner container is 1nserted in the outer container. or bowl of the ultrasonic cleaner, filled with liquid and the articles to be decontaminated are submerged in the liquid. The space bea tween the containers will also be filled with liquid- The.

cleaning apparatus will then be operated until the ultrasonic waves traveling from the outer container into the liquid in the inner container, and the resulting cavitation, have removed substantially all radioactive material of appreciable contamination of ground water. -The use.

of such inner containers likewise greatly facilitates safe handling and transportation of the radioactive material removed from the contaminated articles. 7

An additional step which constitutes a-further feature of the invention greatly reduces, and in practice 'substantially eliminates, any material danger of objectionable effects from the radioactive particles in the cleaning liquid. This is accomplished by mixing with the liquid a material which adsorbs such particles. Such material is advantageously in finely divided form .-and is added after the cleaned articles areremoved from the. inner container. The liquid containing theadsorptivematerial may be agitated, asby shaking tbe container or stirring, to obtain uniform distribution and the consequent removal from the liquid of the greater part of the radioactive substance. When an-inner container with the contaminated liquid therein is buried, leakage is unimportant, since the proportion of contamination in the liquid is so low that its effect in practice will be negligible. It has been found that a'relativelysmall-proportion of the adsorptive material will be adequate for ordinary purposes, a proportion as low as one percent of the amount of liquid by volume having been found to be effective, though larger amounts up to ten percent may be used. Substances that have'been found suitable for use as such adsorptive material include colloidal sulfur,

iron oxides, activated charcoal, halogen salts, manganese dioxide and clays, especially fullers earth. -These substances are of course most effective when introducedin very fine or pulverulent form. The adsorbent used should be selectedv on the basis of the particular radioactive substance or substances that produce the contamination, since each-radioactive substance'is adsorbedmore.

rapidly and-efiectively by-certain adsorbents. For instapce,'f radioactive cesium is adsorbed withnnexpected efliciency by clay, and-especially fuller-s earth. Abrief period of agitation sufficient to maintain complete disper-- sion, normally substantially less than a ;minute, is adequate for effective adsorption; and with proper selection of the adsorbent, contamination can be-substantiallyremoved from the liquid in this manner.

The following examples are given by way of illustra- 7 tion and are not to'be considered as restrictive.

Example I isinserted in the bowl with the upper edge of the con-- 4 Y tainer bent outwardly over the margin of the bowl.

Detergent'liquid is poured into the foil container and liquid, which may be water or detergent liquid, is poured between the'foil container and the bowl. Articles to be. cleaned are placed in the liquid in the container and the cleaner is operated to produce cavitatignin such liquid 7' I An inner container formed of copper foil 0.01 inch,

'thick is inserted in an ultrasonic cleaner. bowl in con-' tact "with the inner face of the bowl, Detergent liquid is pour'edinto the? container and liquid, whichvmay be detergent or water, is poured between thelcontainer and. the bowl, contaminated articles. are placed in the liquid in the container andthe. ultrasonic cleaner is'ope'rated'to produce cavitation around such articles. After the articles are cleaned they are' removed from the liquid and rinsed, and an amountof clay equ'alin volume to five] percent of the liquid in the containeris'introduced into. the liquid and stirred. The top edge of, the container is brought together and bent over to form a seal, after which the container is buried.

' Example 3 The procedure 'of Example 1 is followed toclean 'arti- .cles contaminated with radioactive cesium. After the,

articles are removed from the liquid an amount of fullers' earth equal in volume to one percent of the liquid inthei container is introduced into the liquidand stirred.' The container is then sealed and buried.

Example 4 A-container' formed of a flexible sheet of -'polyethylene, plastic, 0.01 inch thick, is placed in the bowl of' an ultra? 'sonic cleaner and the upper edge of the container is turned outwardly aroundthe bowl margin with the conof-the articles. When'cleaning iscompleted the'articles are removed, the upper. margin of the plasticcontainer;

is brought together and tied or clamped to form a seal,

and the container is buried, 7 7

While preferred for-ms 'of the invention, with variations and examples, have been given, the specific features of the disclosure have been given by way of illustration and not to indicate the fullscope of the invention, which is set forth in the claims.

As it is used herein, the word ultrasonicrefers to V mechanical wave action and does not refer to a wave action limited to any frequency or frequency range within or above the range of audible frequencies. a "I claim: i H

l. The-method of removing radioactive'materialfrom a contaminatedarticle by the action of ultrasonic waves in cleaning apparatusprovided with a bowl containing 7 liquid whichcomprises inserting in the bowl andjli'quid an inner contaiu'en'placing detergent liquidin such container, submerging the article in the detergent, liquid;

transmitting ultrasonic waves from the bowl into the detergent liquid to produce cavitation at 'the surfacejo'f the article, removing the article from the detergent liquid after removal of such'material therefrom, and sealing the detergent liquidhin ,the container in condition for.

disposah- 2. 'The method set forth in claim 'l'in' whichthe container is made of flexible material, and the sealingcomprises folding together the open top portion. of the container. V

1 3. The method set forth in claim 13in, which the eontainer is mad'eof fiexible'material with an open top simi? lar in size to the bowl top, and is held in place during the ultrasonic wave transmission by deflecting such top outwardly across the top of the bowl.

4. The method set forth in claim 1 in which the container is formed of metal foil.

5. The method set forth in claim 1 in which the container is formed of aluminum foil.

6. The method set forth in claim 1 in which the container is formed of copper foil.

7. The method set forth in claim 1 in which the container is formed of flexible sheet synthetic resin material.

8. The method set forth in claim 1 in which the container is formed of flexible sheet polyethylene.

9. The method set forth in claim 1 in which the container is formed of flexible sheet polystyrene.

10. The method of removing radioactive material from a contaminated article by the action of ultrasonic waves in cleaning apparatus provided with a bowl which comprises inserting in the bowl an inner container, placing detergent liquid in such container, submerging the article in the liquid, transmitting ultrasonic waves from the bowl into the liquid to produce cavitation at the surface of the article, removing the article from the liquid after removal of such material therefrom, mixing with the liquid a finely divided adsorbent of the radioactive material, and sealing the liquid in the container in condition for disposal.

11. The method set forth in claim 10 in which the adsorbent is used in proportions of between one and ten percent of the liquid by volume.

12. The method set forth in claim 10 in which the adsorbent is a clay.

13. The method set forth in claim 10 in which the radioactive material is cesium and the adsorbent is fullers earth.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES U.S. Atomic Energy Comm. WASH-408 Disposal of Radioactive Wastes in the U8. Atomic Energy Program, May 17, 1956, pages 1-5.

US. Atomic Energy Comm. WASH-275, Sanitary Engineering Conference, Baltimore, Md., April 15-16, 1954, August 1955, pages 2l52l9, 230 245. 

1. THE METHOD OF REMOVING RADIOACTIVE MATERIAL FROM A CONTAMINATED ARTICLE BY THE ACTION OF ULTRASONIC WAVES IN CLEANING APPARATUS PROVIDED WITH A BOWL CONTAINING LIQUID WHICH COMPRISES INSERTING IN THE BOWL AND LIQUID AN INNER CONTAINER, PLACING DETERGENT LIQUID IN SUCH CONTAINER, SUBMERGING THE ARTICLE IN THE DETERGENT LIQUID, TRANSMITTING ULTRASONIC WAVES FROM THE BOWL INTO THE DETERGENT LIQUID TO PRODUCE CAVITATION AT THE SURFACE OF THE ARTICLE, REMOVING THE ARTICLE FROM THE DETERGENT LIQUID AFTER REMOVAL OF SUCH MATERIAL THEREFROM, AND SEALING THE DETERGENT LIQUID IN THE CONTAINER IN CONDITION FOR DISPOSAL. 